Backup designated router. An elected router in a multi-access OSPF network that acts as the backup if the designated router fails. All neighbors form adjacencies with the backup designated router (BDR) as well as the designated router.

The time within which an OSPF router must receive a Hello packet from an OSPF neighbor. The dead interval is usually a multiple of the hello interval. If no Hello packet is received, the neighbor adjacency is removed.

default gateway

A router to which all unroutable packets are sent. Also called the router of last resort.

delay

The length of time required to move a packet from the source to the destination through the internetwork.

Defines routes by distance (for example, the number of hops to the destination) and direction (for example, the next-hop router) and then broadcasts to the directly connected neighbor routers.

DNS client

Domain Name System client. Communicates with DNS server to translate a host name to an IP address.

DR

Designated router. An elected router in a multi-access OSPF network that sends LSAs on behalf of all its adjacent neighbors. All neighbors establish adjacency with only the designated router and the backup designated router.

DUAL

Diffusing Update Algorithm. EIGRP algorithm used to select optimal routes to a destination.

Link-state advertisement. An OSPF message to share information on the operational state of a link, link cost, and other OSPF neighbor information.

link-state database

OSPF database of all LSAs received. OSPF uses this database to calculate the best path to each destination in the network.

link-state refresh

The time that OSPF floods the network with LSAs to ensure all OSPF routers have the same information.

load

The degree to which a network resource, such as a router, is busy.

load balancing

The distribution of network traffic across multiple paths to a given destination.

M

message digest

A one-way hash applied to a message using a shared password and appended to the message to authenticate the message and ensure the message has not been altered in transit.

metric

A standard of measurement, such as the path bandwidth, that is used by routing algorithms to determine the optimal path to a destination.

MD5 authentication digest

A cryptographic construction that is calculated based on an authentication key and the original message and sent along with the message to the destination. Allows the destination to determine the authenticity of the sender and guarantees that the message has not been tampered with during transmission.

MTU

Maximum transmission unit. The largest packet size that a network link will transmit without fragmentation.

N

NDP

Neighbor Discovery Protocol. The protocol used by IPv6 to find the MAC address associated with an IPv6 address.

network layer reachability information

BGP network layer reachability information (NRLI). Contains the a list of network IP addresses and network masks for networks that are reachable from the advertising BGP peer.

next hop

The next router that a packet is sent to on its way to the destination address.